As a visitor to Prague you probably won't need to use many buses, as they mostly operate in the outskirts of the city. But if you do, they operate much as the trams do. You validate your ticket, which you have already bought, as soon as you get on the bus (if you haven't validated it earlier in your trip). You don't buy the ticket from the driver with one exception.
The main buslines tourists will use are:
Number 100 which runs from the end of the B (yellow) metro line at Zličín to the airport. Exit the Metro Station and you will find yourself in a bus station, turn right and follow the path round to a series of bus stops. The Number 100 runs from one halfway up the rank.You have to buy a special ticket for 16 Czk for luggage over 25x45x70cm (ie hold luggage not hand luggage). A one-day or three-day season ticket includes one item of hold luggage.
Number 119 which runs from the Nasrazi Veleslavin Metro Station to the airport. Follow the signs to the airport bus from the metro concourse. The extra charge for large luggage applies.
The AE (Airport Express) bus is the exception to the rule about not buying the ticket from the driver. The ticket costs 60 Czk, but luggage is free. It runs from the Main Train Station (Hlavní nádraží) to the airport. Follow signs to the bus station which is upstairs from the platforms. It runs every 30 minutes.
Number 112 runs from Nádraží Holešovice Metro station (line C) to the Zoo and Troja Palace.
You can see some lovely old Prague buses at the Prague Public Transport Museum in Prague-Střešovice.